This is particularly relevant since one of the main reasons we're worried about the ad model at all is because it threatens the internet's decentralized nature. Replacing this with a model where you only make payments inside a few centralized organizations would not address this core issue.
That process won't scale to a whole bunch of random, disparate sites especially if you're not heavily invested into them. If you come across a cool blog post and want to tip a bit of money, you'll have to go through the same process, which is just not worth it. And that's before getting into transaction costs!
There have been various services that tried to fix them, but none so far that are convenient and ubiquitous enough to change things. And we really need change: this is, to a large extent, a social problem more than anything. The most important piece for moving away from ads is to make small payment expected and, for lack of a better phrase, socially accepted.
We already have this for small physical goods: it's easy to imagine buying a piece of gum or a magazine at a random newspaper stand. But a similarly small purchase online is both less convenient and less accepted, so it ends up being much less likely. That's the core problem.